Conduit with first fix wiring

[benjy_b]

Hi all,

Whilst renovating our bungalow we will be fitting additional & relocating current sockets and switches.

The lighting wiring runs throughout the loft space and the socket wiring runs below the suspended timber floor.

Can I run conduit to the back boxes with the intention to upgrade and run additional wiring at a later date? (To spread the expense)

What regulations must I consider?

Examples are:

- conduit down to light switches, between a studded wall (with insulation fitted both in the loft and between the stud wall)

- conduit fitted upwards to plug sockets, through 150mm sheeps wool insulation into either a studded wall or "dot & dab" dry lining on external walls.


My limited experience considers issues with fire protection and drafts. But maybe the use of expanding foam or gaskets would help?

 

[endecotp]

How much cost are you really postponing? (How much does cable cost? How much does conduit cost?)
Pulling cables through conduit can be painfully difficult.

 

[ban-all-sheds]

Can I run conduit to the back boxes with the intention to upgrade and run additional wiring at a later date? (To spread the expense)

Yes, but how much will omitting the cables (and sockets if you don't mind gaping holes) save you?

What regulations must I consider?

The Building Regulations - Part P and possibly the requirement to notify in advance.

The Wiring Regulations - strictly speaking not mandatory, but in practice so de facto the way to do things that they might as well be.

Examples are:

- conduit down to light switches, between a studded wall (with insulation fitted both in the loft and between the stud wall)

- conduit fitted upwards to plug sockets, through 150mm sheeps wool insulation into either a studded wall or "dot & dab" dry lining on external walls.


My limited experience considers issues with fire protection and drafts. But maybe the use of expanding foam or gaskets would help?

You must also consider the issues of cable de-rating because of running in thermal insulation.

• http://www.google.co.uk/search?q=cable+installation+methods+17th+edition
  
  
• [wiki]Electrics%3AMCBtoCable[/wiki]
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20070327014113/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html <- Specify 2-core cable and PVC, even if that is not what you are using or it will make invalid assumptions.
  
  
• http://electrical.theiet.org/wiring-matters/28/appendix4.cfm?type=pdf
  
  
• http://electrical.theiet.org/wiring-matters/40/appendix-4.cfm?type=pdf
  
  
• http://www.batt.co.uk/upload/files/4d5.pdf
  
  The references above which are in italics are either out of date or incomplete. The principles are still valid, and you can absolutely learn valid things from them, but you will need to refer to the latest edition of the Wiring Regulations to get the correct data.

If you can get the conduit touching the plasterboard you'd be looking at Installation Method A, but if it'll be surrounded on all sides by insulation I'd say Method 103. If the former you'll get away with 2.5mm² for ring finals, if the latter you'd need to use 6mm², which would be mightily impractical.

 

[benjy_b]

Hi, thanks for the great response.

I shall study those pages/documents thoroughly and come back with any further questions or developments.

 

[ban-all-sheds]

Do bear in mind that you can't really do design like this on the basis of asking whatever questions happen to occur to you - you need to learn the subject in a more controlled and methodical way.

 

[benjy_b]

Do bear in mind that you can't really do design like this on the basis of asking whatever questions happen to occur to you - you need to learn the subject in a more controlled and methodical way.

I'm a logical & methodical person by nature. I dislike paying people to do something I could potentially do myself. Especially when the end result doesn't end how I wanted it. Let's just say I've been let down by varying tradespeople in the past. I'd much rather research and self learn a subject over a period of time and attempt it myself. However, I also acknowledge my limitations.

In this instance, I'm not talking about carrying out any wiring myself. Merely facilitating the process by potentially fitting conduit.

 

[flameport]

In this instance, I'm not talking about carrying out any wiring myself. Merely facilitating the process by potentially fitting conduit.

Have you installed conduit before?
If it's not fitted correctly, it will be impossible to pull wiring in afterwards.

 

[benjy_b]

Have you installed conduit before?
If it's not fitted correctly, it will be impossible to pull wiring in afterwards.

No, however I have got some transferable skills from automotive wiring that I'm sure will come in handy.

I know I've not supplied any images, so your mind will be wondering. But we're not talking endless metres of conduit snaking below my joists.

It would literally be:

- wiring running under joists
- takes 90 degree turn upwards
- ran into some vertical conduit (held in place with clips against joist)
- through 150mm of sheeps wool insulation
- into the connected backbox
- and connected to socket

 

[benjy_b]

Using a very crude description, the conduit would be acting like a periscope for the cable.

 

[Iggifer]

This 90 degree turn is from where to where? You need to follow the safe zones, will be nigh on impossible to blindy push more than one cable down a 90 degree bend you don't have access to. You would at least need to put some draw tape in, then you might stand a bit of a chance

 

[flameport]

- wiring running under joists
- takes 90 degree turn upwards
- ran into some vertical conduit (held in place with clips against joist)
- through 150mm of sheeps wool insulation
- into the connected backbox
- and connected to socket

Singles are used with conduit.
Singles cannot be used outside of conduit
Sheathed cables can be used outside of conduit
Sheathed cables will be next to impossible to pull into conduit with bends.

Conduit will increase the cost and time of installation significantly. It would be cheaper and quicker just to clip the cables to the side of the studs in the wall, even if the cables are not used immediately.

 

[benjy_b]

The cables in our bungalow are currently fitted to the bottom of the joists that make up our suspended timber floor.

We are wishing to insulate said floor.

I'm proposing running conduit through the insulation and so it will sit flush with the joists and connect to the backbox.

The runs of conduit will be perfectly straight and vertical down from the backbox, through the sole plate, through the floorboards, through the insulation, into the cavity. This is where the cable will take a 90 degree turn up into the conduit.

Does that explain it better?

 

[benjy_b]

Conduit will increase the cost and time of installation significantly. It would be cheaper and quicker just to clip the cables to the side of the studs in the wall, even if the cables are not used immediately.

Thank you for the response.

I feel I've not properly explained our intentions with the renovation.

We are not wanting a cheap or quick solution. We intend to stay in this property for a long time. Hence, taking our time & learning as we go along.

We're also trying to maintain a high standard and use premium products where we can.

 

[ban-all-sheds]

I'm not talking about carrying out any wiring myself. Merely facilitating the process by potentially fitting conduit.

So you know all about conduit factors - how to calculate them given the lengths, numbers of bends, elbows, sets, double sets etc? And the related cable factors for the numbers, sizes and types of cables which will be in them, allowing for the sizes you need given the installation methods, grouping de-ratings and so on, and can therefore come up with a design for a conduit system which an electrician can actually use in the future to draw the appropriate cables in?

Or are these things which you've not asked about, not because you know all about them, but because you simply have no idea that they even exist?

This is the "unknown unknowns" problem with thinking you can do work like this by asking as you go along about the things you happen to realise you don't know. You end up still not knowing really important stuff because you never realise that you don't know it.

I'm proposing running conduit through the insulation and so it will sit flush with the joists and connect to the backbox.

What will happen if it turns out that the conduit size you need is not compatible with backbox knockouts?

Do you even know what size cables you will need for this installation method? If you are planning 32A ring finals, and you end up needing 6mm² conductors, have you thought about the fact that finding sockets which can take 2x6mm² might limit your choices? And cause your electrician to curse you.

The runs of conduit will be perfectly straight and vertical down from the backbox, through the sole plate, through the floorboards, through the insulation, into the cavity. This is where the cable will take a 90 degree turn up into the conduit.

Have you thought about how the numbers and sizes of conductors you will need will be able to be pulled through something like that without being damaged?

I'm a logical & methodical person by nature. I dislike paying people to do something I could potentially do myself. Especially when the end result doesn't end how I wanted it. Let's just say I've been let down by varying tradespeople in the past. I'd much rather research and self learn a subject over a period of time and attempt it myself.

Nothing whatsoever wrong with that.

But this is not a trivial job, and I can assure you that it involves knowing far more than you think it does. Asking questions here can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.

 

[benjy_b]

I fear we're all over complicating the conduit here. As I mentioned before, there is no complex runs of conduit.

One of my examples specified a cable run up from under the first floor cavity.

This would include:

Joist height - 225mm (with 150mm insulation)
subfloor thickness - 25mm
sole plate thickness - 38mm
minimum socket height - 450mm

So, a potential conduit/trunking length of 738mm (no bends, no elbows, no sets or double sets... just a straight run up to a backbox, through the various components)

Here's an example of what I mean




Essentially, my question is, "Can I run cable through conduit, through insulation?"